1.3 Generate an Error Message

Partly so you won't worry if you do it accidentally, we will now give
a command to the Lisp interpreter that generates an error message.
This is a harmless activity; and indeed, we will often try to generate
error messages intentionally. Once you understand the jargon, error
messages can be informative. Instead of being called “error”
messages, they should be called “help” messages. They are like
signposts to a traveler in a strange country; deciphering them can be
hard, but once understood, they can point the way.

The error message is generated by a built-in GNU Emacs debugger. We
will `enter the debugger'. You get out of the debugger by typing q.

What we will do is evaluate a list that is not quoted and does not
have a meaningful command as its first element. Here is a list almost
exactly the same as the one we just used, but without the single-quote
in front of it. Position the cursor right after it and type C-x
C-e:

(this is an unquoted list)

A *Backtrace* window will open up and you should see the
following in it:

Your cursor will be in this window (you may have to wait a few seconds
before it becomes visible). To quit the debugger and make the
debugger window go away, type:

q

Please type q right now, so you become confident that you can
get out of the debugger. Then, type C-x C-e again to re-enter
it.

Based on what we already know, we can almost read this error message.

You read the *Backtrace* buffer from the bottom up; it tells
you what Emacs did. When you typed C-x C-e, you made an
interactive call to the command eval-last-sexp. eval is
an abbreviation for `evaluate' and sexp is an abbreviation for
`symbolic expression'. The command means `evaluate last symbolic
expression', which is the expression just before your cursor.

Each line above tells you what the Lisp interpreter evaluated next.
The most recent action is at the top. The buffer is called the
*Backtrace* buffer because it enables you to track Emacs
backwards.

At the top of the *Backtrace* buffer, you see the line:

Debugger entered--Lisp error: (void-function this)

The Lisp interpreter tried to evaluate the first atom of the list, the
word ‘this’. It is this action that generated the error message
‘void-function this’.

The message contains the words ‘void-function’ and ‘this’.

The word ‘function’ was mentioned once before. It is a very
important word. For our purposes, we can define it by saying that a
function is a set of instructions to the computer that tell the
computer to do something.

Now we can begin to understand the error message: ‘void-function
this’. The function (that is, the word ‘this’) does not have a
definition of any set of instructions for the computer to carry out.

The slightly odd word, ‘void-function’, is designed to cover the
way Emacs Lisp is implemented, which is that when a symbol does not
have a function definition attached to it, the place that should
contain the instructions is `void'.

On the other hand, since we were able to add 2 plus 2 successfully, by
evaluating (+ 2 2), we can infer that the symbol + must
have a set of instructions for the computer to obey and those
instructions must be to add the numbers that follow the +.

It is possible to prevent Emacs entering the debugger in cases like
this. We do not explain how to do that here, but we will mention what
the result looks like, because you may encounter a similar situation
if there is a bug in some Emacs code that you are using. In such
cases, you will see only one line of error message; it will appear in
the echo area and look like this:

Symbol's function definition is void: this

The message goes away as soon as you type a key, even just to
move the cursor.

We know the meaning of the word ‘Symbol’. It refers to the first
atom of the list, the word ‘this’. The word ‘function’
refers to the instructions that tell the computer what to do.
(Technically, the symbol tells the computer where to find the
instructions, but this is a complication we can ignore for the
moment.)

The error message can be understood: ‘Symbol's function
definition is void: this’. The symbol (that is, the word
‘this’) lacks instructions for the computer to carry out.